Optical combination.



fdl

0. EPPENSTEIN.

OPTICAL COMBINATION.

APPLwATxoN FILED SEPT. z2, 1909.

Patented May 30, 1911.

2 SHEETS-'SHEET 1.

0. EPPENSTBIN.

OPTICAL COMBINATION.

APPLIOATION FILED snPT.zz,19o9.

Patented May 30, 1911.

2 SHEETS-SHEET 2.

UNTTED sTATEs PATENT oEEIcE.

OTTO EPPENSTEIN, F JENA, GERMANY, ASSIGNOR TO THE FIRM OF CARL ZEISS, 0F

JENA, GERMANY.

OPTICAL COMBINATION.

esseci?.

Specification of Letters Patent. Patented May 30, 1911.

Application filed September 22, 1909. Serial No; 518,974.'

i To all whom it concern.'

; shoul ,tion

Be it known that I, Orro EPPENSTEIN, a citizen of the German Empire, residing at Carl-Zeiss strasse, Jena, in the Grand Duchy of Saxe-lVeimar, Germany, have invented a. new and useful O tical Combination, of which the followin 1s a specification.

The invention re ates t0 awell known combination of a front system of plane reflectors with twov hinder telescope systems. The system of plane reflectors of that combinatlon brings two parallel pencils of parallel rays to a smaller orlarger distance between each other than that in which they impinge on the combination. Each of the two telescope systemsreceives onerof the pencils. The systemof plane reflectors is always so arranged that its members have the same prlncpal plane of reflection, which term be understood to denote that plane to which the encil, both when entering the member an when emerging from it, is parallel. For the members of the reflecting .system diil'erent forms are used: firstly, a

simple reflecting surface-usually embodied in an isosceles right angled prism having a totally reflecting hypotenusal surface-secondly, a group or two or three reflecting surfaces having together a constant angle of del -flection of 903-usually realized by an optical square prism. In the members of the reflecting system, a simple reflecting surface is often replaced by a ridge surface for the purpose of influencing the pos1t1on of the Image. The number of the members of the reflecting `system is two or four. In most cases, the system is composed of members of the same kind. As to the system of two members, two different arrangements are to be distinguished. lIn the one arrangement not only the distance between the two )encils is altered, they also undergo a inal chan re of direction corresponding to a rotay 900. The two members ot this reflecting system have the same direction and may therefore be called a pair of members. Each member receives one of the pencils and deflects it by 90, the final distance between the pencils being determined by the distance as selected between the two members in the original direction of the pencils. In the second arrangement of the reflecting system of two members, these two members have opposite direction. One peucil is only exposed tothe action of its telescope system. The other pencil is deflected by the two members successively, each time by' 90, at first in one, thereafter in the oppos1te direction. In this second case, the two members of the reflecting system, because ofttheir operation, maybe called a double memer. flected pencil is parallel to the original one, and the amount of enlargement or diminution of the distance between the pencils is determined by the distance the two members have between each other in the direction of the pencil passing from one to the other member. If the distance between the pencils is enlarged or diminished only partly with the one pencil, but for the rest with the other pencil y means of a second double memberfthe reflecting system of yfour members has been made use of. This s stem hence displaces one pencil to one si e by means ot one double member and the other pencil to the other side by means of the other double member. The sum of the two displacements represents the enlargement of the diminution of the distance between the pencils. No other arrangement of the reflecting system of four members requires practical consideration. The two telescope systems may have more or less components in common. When their two image fields lie closely together or even one in the other, the ocular is common to both. When in addition thereto, the objectives of the two systems lie closely together, they are in most cases constructed as halves of a single ob,- jective.

To the optical combination as considered, all those telemeters belong, the objectives of which having the axes parallel either lie together with less distance between them than the length of the base line amounts to or are united into a single objective. The system of 4plane reflectors reduces the distance between the two pencils, which is Originally equal to tbe length of the base line, to that amount in which they can enter the objectives or the single objective. All of the three forms of the reflecting systemthe two of two members and that of four members-Ware in use with the said telemeters. 'lhe combination of the two telescope systems with a front system of plane reflectors in any one of its three forms is further produced, when the adjusting device. the most The final direction of the twice deiml'iortant constructions of which are de- 116 scribed in the German specification 165510,

. is placed in front of any telemeter contain.- singqthebase line, for example, of one of the Vkind j'ust referredt'lvv The 'system of plane'reflectors of this adjustingv device enlarges the -distance between the two pencilsto the length of'. the base lline. It is the reflecting-system of' the combination yhere -dealt with, and the reflectingsystem which `might Ibe employed in the telemeter should only be considered as supplemental to the [two telescope` systems of the telemeter.

Through t-he' action of the reflecting sys# -tem,-tl1e pencils are, in general, not again l5 directed exactly parallel., Behind the refleeting system, they have therefore a slight inclination to each other. This small deviation, .reduced tol the principal plane of' reflection, may 'be called error of reflection'.

..20 When the vreflecting syste-m consists. of al pair of members or a double member, the error of reflection is'A e ual to the dieren'ce of deflection of the pair of ymembers or the double member, that is to say, equalto thev difference of the angles,` by which the -two members deflect the incident pencils. When the kreflecting system is com osed, of vtwo double members, the error o .reflection is equal to the algebraic differenceof the differences of deflection of one and the other double member.v In order to enable the telemeters and .the adjusting device, to comply with their purposes in a perfect manner,'the error of reflection ,must be invariable and that for the following reason.

With all forms of the combination as considered, the pencils are induced to form each a real image point in one 'and the other ocular field of the telemeter respectively. 40 The distance between these two image points,

`measuredin the direction of the base line,`

is a standard for adjusting the telemeter. But when the error of reflection changes, this distance varies correspondingly, whereby the telemeter or the adjusting device itself is deranged.

/ As experience has shown, the reflecting system accomplishes in none of its forms therequirement of invariability of the error of reflection. There' takes place a gradual alteration of this error, one time in one' direct-ion, another in the other direction.

lVhen in a system of two members simplereflecting surfaces are employed as members. the change'results from a slow alteration of the angle formed by the two reflecting surfaces of the pair of members or the doubleI member. When the members consist of groups of reflecting surfaces, in each group the' angle between neighboring reflecting surfaces varies slowly, hence also the angle f of deflection proper to this group, but in general in one group of the pairof mem-' bers or the double member to another extent than in the second group. Similarly,

in reflecting ystems composed of two double members, `the change ,of -thedifl'erence of deflection is in general different lwith. the one and with the vot'herdouble member, so that valsothe error of reflection, being the algebraic difference of the two differences of deflection, varies. No means have-become pubhc 'to obviate the r1se ofslow variations of the error'of reflection. y

J.The object of the present invention is tov 75 render the alterationl of the error of reflec i tion, when arisen, capable of being'l ascer tained in a simple manner as to'both sign and magnitude. l For this purpose the mem'- -bers vof thefsystein'of plane vreflectors are ,-80

mounted ,in such a manner that they can be re-arran'ged so asto produce an error of re? f fleet-'ion of the same absolute value as in the formerV case, ut `of opposite sign.. After j such re-arrangement the pencils converge by as much .as thefypreviously diverged, or. inversely. .On rst employing the 'one Aarrangement yofthe `members "of 'the system, and then immediately-th`at is to say, before the gradual alteration of the error of 90 vreflection has had time to become perceptible-employing lthe ,second arrangement, the

ybeing given, then the mounting of the re'-` flectors which allowsof a re-arrangement with the effect above stated differs in gen eralaccording as the system consists of a pair of members, a double member or two double members, and it further differs'` ac-.

cording as simple reflecting surfaces ors 11Gl roups of such surfaces represent the finemers. In some cases, several arrangements. are possible, particularly when a change in the position of the incident pencils lis admitted. l i

In the annexed drawin Figure 1 Vis a diagrammatic plan view o a telemeter comprising the combination as improved by the resent invention, the reflecting system belie ing formed bv a pair of members. Fig.l 2 is 120?l a dia am o the same telemeter after the 'reflecting system has been rearranged.'

F ig. 3 is a diagram of a `formy of the` 1mproved combination, which comprises an adv justing device and a telemeter, the former 12b consisting of a reflecting systemy formed by a pair of members and the latter being represented by its front parts only.` Fi 4 is a diagram of the same combination a er the adjusting device has been ire-arranged. `L30 two double members. Fig. 22 is a diagram 5. is 'a diagram of a combination similar to Fig. l is adiagram of the same combina-V tion after re-arrangement. Fig. l1 is a diagram of a combination differing from those of Figs. 3. to 10 in that the reflecting system is formed by a double member. Fig. 12 is a diagram of the same combination after rearrangement. Fig. 13 is a diagram of a combination similar to that of Flgs. 11 and is an axial-section throu h an a 12. Fig. 14 is a diagram of the same combination after re-arrangement. Fig. 15 is a diagram of a telemeter comprising the improved combination, the hinder parts being omitted and the reflecting system formed by a double'` member. Fig. 16 is a diagram of the same telemeter after re-arrangement. Fig. 17 is a dia aml of a combination similar to that .of Figs. 11 and 12. Fig. 18 is a diagram of the same, combination after rearrangement. Fig. 19 is a diagram of a combination difl'erlng from those of Fi 3 to14 and Figs. 17 and 18 in that the re ecting system is formed by two double members. Fig. 20 is a diagram of the same combination after re-arrangement. Fig. V21 is a diagram of a telemeter comprising the improved combination, the hinder parts being omitted and the reflecting system formed by oli the same telemeter-after re-arrangement.

Fig. 23 is an axial section through a telem` eter comprising the improved combination, the reflecting system being formed by a pair of members. `Fig. is the samevsection ater re-arran ement, the hinder art of the telemeter ing broken away.- ig. 25

just'ng device adaptedsto be place in front of a telemetersoas. to form the improved combination. 26 isthe-"same section after -rearrangement. y

In-the telemeter of Figs. 1 and 2 a terrestrial ocular a is 'common to both telescope systems, whereas one half of the objective b apperta'ins t'o one and the other to the other telesco system. The measuring device which longs to one telescope system is indicated by the sliding refr-acting prism o, which isarranged behlnd the half objective of this telesco system. The two image fields lie in eac other, their locus being indicated by the diaphragm d0. The reflecting system embodies the hase line of the telemeter and is formed by a pair of members, the membersdland zlfofy which are isosceles right angled prisms which reflect totally. The two prisms d* and d'1 are fixed in a commony casing e havingentrance openings on l.bothsides. From the arrangement of Fig.

1,"thatof Fig. 2 is attained by changing the position of the prism casingr e with the effect of a rotation by 180 in. the principal plane of reflection (in the Vplane of the drawing). The two pencils impingingl/on the prisms with parallel rays and wit' a distance between each otherequal to the length of the base line are represented each by a single ray. According toFIg. 1, the prism d deflects the left pencil by the an le u), the prism d' the rifght one by the ang e u'. Hencethe error o reflection amounts to tal-u. ond arrangement, Fig. 2, the left pencil is deflected through. alf by 'u' and the right one through d byl .'v. The error of reflection is therefore v-fvl. When u u, in Fig. 1 the two pencils enter their halves of the objective with divergent principal rays. As in that case at the same time ful fv", .in Fig. 2 the two pencils enter their halves of the In the secsoy objective with convergent principal rays.

When, inversely, u' uand in consequence thereof v v, convergence occurs in Fig. 1, but divergence in Fig. 2. The two errors of reflectlon ul-Ju' and ufv1 have unlike signs in both cases. Moreover, according to the law of reflection, each of the two errors of reflection is twice as great as the difference in direction of the two reflecting surfaces. The two errors of reflection are thus of equal value, having onl opposite directions, z'. e., unlike signs. ence, the second arrangement of the reflecting system d* d accomplishes the above sti ulated condition. With the reflections in ig. 2, where the pencils im inge externally on the reflecting surfaces o the prisms d d', the greater portion of the light is lost byenterin the prisms. The two image po1'nts,'-whic are roduced in the plane of the diaphragm a rom the pencils proceeding from an infinitely distant object point, have thus a comparatively low luminosity. On that account, the arrangement of the system ofv plane reflectors accordin to Fig. 1 is to be employed in measuring lstances and the arran ement according to- Fig.'2 only in conL trol in the. error of reflection.

In t e combination accordingl to 3 and 4, the pair of membersconsists again of simple reflecting surfaces. The telemeter is indicated by itstwo objectives b and b' and the sliding refracting prism c 'of its measuring device. In place of the reflecting prisms of the first example, which in one arrangement act with total reflection, but in the other only to a small extent, here two reflecting strnctures'are made use of, which present equally great reflections in both arrangements of the system. The member f consists of two ,plano-parallel plates, the member f of two isosceles right angled reflecting prisms. In both members, the two like parts are connected together with the silshown by provided' withexit opemngs onboth sides.

The difference between the two arrange- 4--ments of the reflecting system,'the arrangeV with fthe upper halves of thefobjectives, f being for this reason inthe figures, fmight be considered as horizontal' sections through" the plane of l the objective axes," dottedv lines.-` kThe casingl c 1s-v `gver'coa'ting's in contact, or a .,single' layer of v silver orfa vstratum of air -1s provided be-y tween thetwdpar'tsa The incident-pencils' having parallel; rays proceed from acollu'nator. or a distant object' point vand 'travel closely one above. the other,- 4'afs indicated .by

onearrow line, but vwhich -has a double feather. The member f receiving the lowerV pencilisy arranged level with jhe -\lower j halves of the-g-objectives. The member'f. .A

receiving-the upper pencil is: arranged level-- ment of Fig. 3 and that of Fig. 4, corre?" spends again to 'arotation by 18( in the prmcipalf plane yof reflection. Hence, in this `case likewise the. proof isapplicable, given for the pair of members consisting of sim le" reflecting surfaces according to Figs. 1 an 2,l

3 that the errors of reflection of the two arqrangements of the reflecting system are equal but have unlike signs.

justing device consist yof optical square prisms g and.' g, to which, the angles of deflection lwlfand wr` are proper. The second arrangement of the-reflecting system shown by interchanging the prisms.' By this inter'- change, the angle fw is likewise exchangedv In 5and 6 themembers ofthe ad l l in Fig. 6 maybeproduced from that of Fig. 5

for the angle w?, so that the error of reflection 'wl-wr. is transformed into the y other 'wf-w1. rvThe samey arrangement of -the prisms is obtained' lby shiftin without changing its level, be ore the other objective. f

each prism In the example of and 8, as inv Figs. lrand 2, simple reflecting prisms djandd' serve as members.) Thearrangement of the prisms shown' in Fig. `8 may bederived fromthat of 'Fig'. 7 through rotating the prism casing eby 180"v about an axis parallel j to vthe incident pencils. Thisl second ar- Ara ngement is vonly effective, When-as shown in Fig. 8--pencils `are received which pro- Leed m the'right hand, in other words only when on theri'ght hand an opening for the entrance of pencils is provided and as set forth for the left: hand side' with reference to Figs.- -3y and-4,either a collimator can be attachedior a suitable :distant object point-exists. The errors of reflection are equal ,in magnitudefor the same reasons as in the two first examples. Theyhave also unlike signs, though there exists this difference from those examples, that the angle fv lieson the leftand'the angle e' on the right hand'.v The inversion o'f't-he signswhich` occurs when the prisms are re-arranged, re-

Vseafarer sults in this case lfrom the circumstance that, in consequence of the inversion of the mcldent pencils, the' angles of deflection become situated on vthe other side ofthe emerging principal rays, so sign. In Figs. 9 "and 10 optical square prisms are the components of the pair' of members. Changing from one arrangement to the 'other is, according vto the drawing, connected with a change in level of the prisms.

In ythis `case the second arrangement 1s pro'- ducedthrough a rotation of each prism by 1,805v about the axis of its objective. The sam''re-arrangement, but Without a change that they change their -in-levelbf the risms, is obtained by rotat-r. v

ingach prism y 90"y vthe principal plane oitv reflection. In this example also, the correct effect rof .the rearrangement is produced {byinverting the direction of entrance of the pencils.

In-the examples Figsi11 to 18, reflecting systems consisting of a double member are employed. In Figs. lland 12 the double member is composed of simple reflecting tive-bl. lVhen the prism casing e is rotatedI byfv180o' about an Aaxis directed on Va level with the objective axes from 'one to the otherfprism, the arrangement of thereflect` ing system as shown in Fig. 12 is attained. It is seen fromfthis figure, that immediately above and below the axis of the objective b vtwo incident parallel pencils are available,

the vlower one of which is led through the prismsto the objective br. In both arrangements,- the error of reflection depends onlyV upon the double reflection of the one pencil, the `other pencil undergoing no change of direction. 'Hence the error of reflection is in Fig.v11: ur-ul, and in Fig. 12: 'v1-'v Since again, asin the first example, UL-vfi is equal to 'ul-uf, the two errors of' reflection differ from one' another bv the sign only. The example of Figs. 13 and 14 differs from the preceding oneby the double member being composed of optical square prlsms. The transformation of one arrangement of the reflecting system into th(` other may be effected through a rotation of both prisms `by 180o about an axis, which is directed from one prism to the other in level with the objective axes. The same re-arrangement, but without a change in level, and tire same effect is attainedfwhen each prism -1s separately rotated by 90 in the principal lplane of reflection. The error of reflection, is in the first arrangement of they,

prisms unf-fw' and in the second tLUL-w'.

" Inl the-telemeter of' Figs. y15 and 16, the two telescope systems are united similarly as in thel example Figs. 1 and 2, the only difference being that in the resent case the objective isI to be divided t rough a horizontal diameterl into the two halves belonging to dl'. A rotation of the prism casing c by 1230"y in the principall plane of reflection ro-y duces from the one arrangement the ot er. If optical v uares bethe components of the double mem er, their interchange would result inthe correct rearrangement. As to the errors of reflection, the same applies as in the example Fi .p11l and 12, when simple reflectors are use and the same as in the example Figs. -13v and 14, when optical squares are em loyed. The case of Fi s. 17

and 18 differs rom those dealt with in i 11 and 12 as well as 13 and 1.4 in that t e position of the incident pencils is not to be altered. Since optical square risms are chosen as members of the reflecting system, transforming Vone arrangement into the other requires that the prisms are inter` changed, that is to say, that each prism is brought to. theplace and the position of the other. ,This interchange corresponds to a rotation of the double vmember by 180 in the` principal plane of reflection. The same rotationv characterizes the correct re-arrangement if the members have only. simple reflecting surfaces. The error of reflection in the arrangement Fig. 17 is wf-sw, and in the arrangement Fig. 18 it is lw-w.

In Figs. 19 and 2,0 the incident pencils, which mzv be produced in a collimator, lie closely'sl e by side. l According toy Fig. 19, the left pencil, passes through" the simple reflectin prisms d, and d, forming the left dou le member and undergoes in this way the contrary4 deflections u1 andl ula. The 'ri ht pencil passing through the prisms d", an d", under ees the deflections u', and

The error o` reflection is The rearrangement of the reflecting system, Fig. 20,-mig ht be produced through a rotationY 'ofthe yprism Casingsy e and, e by 180 in the principal plane of, reflection. The error of reflection is'. hereby changed to ,In accordance with former explications 'uk-m", 1s equal te u'fnu, and vff'v', equal distance double member is composed of two optical y one of g", a V Q-rangemeutbf Fig. 21 1nto that of Fig-22, each double member may be rotated by 180 lto uff-ug. Hence, the error of reflection of the arrangement in Fig. 20 differs from that in Fig. l19'by the lsignonly. v

In the telemeter shown in Figs. 21 and 22 the two objectives lie closely side by side behind theY two double members, which reduce the distance between the incident pencils from theA length of the base line tothe between the objectives', Each squares, the left one of gli and 2, the right and (f2. To trans orm the arin the principal plane of' reflection or the left members interchanged as well as the right ones. The error of reflection, in Fig.

and that in Fig. 22 is wl2"wli wr2+wrn yso that both differ in si only.

In the telemeter of igs. 23 and 24, the reflecting system which serves as the horizontal baseline is not placed immediately in front of the objective b, as in Figs. 1 and 2, but between both a simple reflecting prism h is located. As in the example )ust re- -ferred to, the reflectin system is a pair' of "members composed o two isosceles ri ht angled reflectin prisms d* and df, whlch have totally re ecting hypotenusal surfaces and are' fixed in a casing This casing has entrance openings on both sides abreast of the prisms and is rotatable about a vertical p axis 'j in the main casing jof the telemeter, ya. snap device 2' j1 being employed to fix the casing-c", after it has been brought, through a rotation by 180, from the position of Fig. 2,3v in to that of Fig. 24, or inversely. The reflecting system is, moreover, providedA with a so.cal edf compensator composed of two refracting Vprisms c and le?, whichV are `geared together for equal, but opposite rotations and so arranged that the pencil system passing through them is deflected in the horizontal plane. Its Object is to comensate the existing error of reflection. In ig. 22 thesaid compensator influences the pencil system deflected by d', in Fig. 24 it acts tothe same extent, but in opposite direction (the sign of the error of reflection is likewise contrary 'to that in Fig. 23 on the pencil system proceedin from d'. mg the o erating whee k of the prisms k1 and Ic, t edegree of deflection produced `can be regulated. As may be understood from Fig. 28, the pencil system reflected by d enters the left half of the objective, that reflected by d' the right one. In order that the two images of the landscape-produced by the objective b from the two pencil systems in the plane of the vertice y turnwires i producing the right image. This prism serves at the same time as the optical com,-

.ponent of the measuring device, whichinaddition-thereto comprises a fixed scale m., a slide l (carrying the prism Z0 and an'index Z1), "and a screw n for displacing the slide l; A vterrestrial ocular 0 renders the optical l equipment of the instrument complete. For

measuring finite distances, the measuring device is to be operated, from the position shown where the index Z1 points to the scale stroke oo, in such a direction that by approaching the prism Z to the wires h1 hr the right image of the object point is moved' to the left one. The amount, b which the distance between the right an the left imagel of ythe'object pointisdiminished, is proportional to the displacement of the prism Z0.

When this distance has become equal to the distance between the wires kl and h (equal vto the-distance between the` images of an infinitely distant object poi-nt when the measuring device is set to oo as shown) the displacement of the right image corresponds to the angle formed by the two pencils which travel from the object point to the prisms d I andV d?. This .parallactie angle can therevfore immediately be measured -by the displacement" of the prism Z0. The scale m',

however,instead o f being provided with an equal division for -the parallactic angles carries an unequal one for the distances o the object, which are'determined on the one side -`by those angles andl onthe `other side 'by the length of'thebase'iine. v When the instrument is directed so that the' left image of the, object point coincides with the wire l v -`L1,.the distance-can be read from the scale m las' soon as the right image of the object point iscoveredby the wire 71,'. IWhen the Y. observer desires to test, whether-the error of `reflection of'v the reflecting system, -formerly brought to zero by means of the compensator ckz, has changed, the casing z' is to be, rotated from the `position Aof Fig. 23 to'thatfof Fig. 24; If the right imagel ofv theobject point is again covered by the wire h', theerror of reflection is still unaltered.

But if this image lies on the rightior the left'side ofthe wire it', the change' ofthe error of reection so indicated is neutralized by operating, the compensator,"until the ri ht limage of the object point has traveled ha4 fgthe way to the wire h. The device hasv thus' been"readju`sted, as the'position of the cease? -the reecting system be not provided with a compensator, two settings of the measuring device are required for determining the distance of an object,- both causing -the right image to coincidewith the wire h', but one effected with the position of the casing z' shown vin Fig. 23 andthe other with that of Fig. `24. As the parallactio angle belonging vto the object point is, in the one so case, diminished by the error of reflection ofthe pair 'of members, in the other. case,

enhanced by the same error, the median position between thev two ositions of the index Z1 as set presents the. istance of the obJect point.

The adjusting device according to Figs. 25 and 26 is designed to be placed, say on a stand, inv front of any telemeter, for example that according toFig. 23. The device is composed of two optical square prisms g1 and g having angles of deflection of about 90', and forming a pair of members and of 'a collimator, lthat is to say, a collective lens p and a mark g in its focus. This mark may `consist of a small a erture as shown. The casing 1' of the co1 imator islrigidy connected with the main casing s of the a j usting device carrying the prisms g1 andyr by means of their mountings t1 and tf. The exit openings s1 and' 's' of the ad]ust1ng device are supposed to be directed toward the entrance openings of the telemeter. The prisms gl and are arranged with different level so that t eleft one receives the lower,

and the right one theV upper pencil of parallel rays reducedI by the eollimator. The

`prisms d e ect the pencils through the openings s* and '8" into the left'andl the lright tele-` scope of the telemeter. The mountings t and t", each together with .its prism, can be interchanged.` After such interchange, the` left prism receives the upper and the right one the. lower pencil, but. this does not iniuence the distance between'the two real 1,115 kimages ofthe mark g of the collimator,`

lwhich are formed from these encils in the telescopes of the telemeter. his distance between the mark images depends, on the contrary, only upon theerror of reflection '120 of the pair of members gl gf.v If the error of reflection is equal to zero, the mark g represents an object oint having the parallactic angle zero, tllat is vto say, an infinitely distant object point, and the distance between the images 1s not altered by thev interchange ofthe prisms.- Furthermore, if

for example the telemeter is provided in each image field with a fixed measuringwire, as h and la, in `Fig. 23, and, in addition ioo Ithereto, with a measuring device in the right A hf. When there exists an error of reflection,`

itspsign changes by interchanging the prisms, and in consequence thereof the distancebetween the images isy likewise altered. The' mark represents, in one case, an object point of finite distance, the parallactic angle of which is equal to the unknown error of reflection; in the other case, an object point, which lying behind `the telemeter has the same parallactic angle as the first object point, but with opposite direction. In the first case, the distance between ,the images exceeds, in proportion to the parallactic anigle, the distance between the images of an in nitely distant object point; in the second y case, it`1s by the same amount less than the latter distance. Hence, thetelemeter is correctly adjusted, when the index of the measuring device,in the setting ofthefirst case, deviates by as much fromithe scale stroke oo toward the side of the finit distances as, in the setting of the second case, in the opposite direction.

` claim:

1. The combination, with a front system of plane reflectors which consists of an even number of membersy arranged with a com- Ato change the mon principal lane of reflection and so as dlistance between two arallel pencils of parallel ra s, oftwoV hin er telescope systems adapte the pencils, plane reflectors being mounted so as to allow a re-arrangement, after Awhich thev error of reflection of the .system has the samel value, but the contrary sign. y

2. The comb1nat1on, with a front systeml which consists of an even number of optical each to receive one of the members of the system of' squares arranged with a common principal plane of reflection and so as tochange the distance between two parallel pencils of parallel rays, yof two hinder telescope systems adapted each to receive one of thepencils, the optical square prisms being mounted so as to allow a re-arrangement b interchange. 3. Adevice for adjusting telemeters, comprising a system of plane reflectors which consists of an even number of members arranged with a commonprincipal plane of reflection and so as to change the distance between two neighboring pencils of arallel rays to an amount `equal to the lengt of the base line ofthe telemeter, the members of the system of plane reflectors being mounted so as to allow a re-arran ement, a ter which the error of reflection lo the system hasthe same value, but the oontrar si 4 t. OTTO E P NSTEIN. Witnesses:

PAUL KRGER, /Fnrrz SANDER. 

